Neuroblastoma, one of the most common forms of childhood cancer, has been treated with only limited success with various antimetabolites used singularly, sequentially or in combination. This tumor, cloned from the mouse, has been adapted to tissue culture, providing an exquisitely sensitive and quantitative experimental system to test the efficacy of new chemotherapeutic drugs, emphasizing a series of folic acid analogues and other inhibitors of pyrimidine and purine synthesis. A major goal has been the isolation of specific drug resistant populations and characterizing this resistance at the transport and enzymatic levels, emphasizing dihydrofolate reductase and thymidylate synthetase. We propose to evaluate a new antifolate which uniquely inhibits both of these critical enzymes, to assess the relative difficulty with which neuroblastoma may be selected for resistance to this drug, to contrast the mechanisms of resistance of this and related drug resistant populations, to determine the homogeneity of these mechanisms, and to test further compounds which might act synergistically with those already examined in enhancing, and preventing the resistance to, this cytotoxicity. These studies are intended to provide a better understanding of the patterns of drug resistance, allowing more efficient chemotherapy regiments to be formulated. BIBLIOGRAPHIC REFERENCE: Baskin, F., and Rosenberg, R. N. In Vivo and In Vitro Studies on Cytotoxicity and Resistance to Antineoplastic Agents in Cultured C-1300 Neuroblastoma. In Press. Drug Treatment Reports, 1977.